1. Field of the Invention
The present invention relates to an infrared sensor and a method of producing the same. More particularly, the present invention relates to a small infrared sensor and a method of producing the same.
2. Description of the Related Art
Generally, an infrared sensor detects intrusion of a person, fire, and other subjects, by utilizing a pyroelectric effect provided by a pyroelectric element mounted in the sensor. The pyroelectric effect is a phenomenon that involves variations in temperature of the pyroelectric element occurring when infrared emitted from a person, fire, or other subjects, enters the pyroelectric element. The variation in temperature modifies spontaneous polarization of the pyroelectric element, and electric charge is generated corresponding to an amount of modification of the spontaneous polarization.
Output of the pyroelectric element varies corresponding to the quantity of electric charge generated due to the modification of the spontaneous polarization of the pyroelectric element. The infrared sensor processes the variation of the output with a circuit to detect a subject.
An infrared sensor of this kind includes supporting portions disposed between a base and the pyroelectric element at both ends or four corners of the pyroelectric element so that the pyroelectric element is spaced from the base by a predetermined distance (Japanese Unexamined Patent Application Publication No. 8-35880 (Patent Document 1)).
In this type of infrared sensor, heat flow may vary in the vicinity of an electrode provided at the pyroelectric element if positions of the pyroelectric element and the supporting portions, or positions of the supporting portions and the base are deviated. When infrared enters the pyroelectric element, a temperature of the pyroelectric element may vary due to the variation of the heat flow, and thus, the output of the pyroelectric element may not be constant with respect to the amount of incident infrared.
To address this problem, a method of producing an infrared sensor is disclosed in Japanese Patent No. 3209034 (Patent Document 2) which includes the steps of printing a conductive paste at both ends of a back surface of the pyroelectric element by screen printing and solidifying the conductive paste to provide supporting portions.
In Patent Document 2, to increase the height of the supporting portions, the printing and solidification are repeated several times to increase the thickness of the supporting portions.
However, the screen printing may cause unevenness of the printed surface instead of a flat surface. If the printing is repeated on the primarily formed, uneven printed surface, the height of the finally produced supporting portions may be uneven depending on the type of sensor to be produced. Accordingly, the dimension of a space (gap) between the pyroelectric element and the base may be uneven.
Heat generated at the pyroelectric element is also transmitted to the space. If the dimension of the space is uneven, the correlation between temperature variation of the space and that of the pyroelectric element may be distorted. In addition, since the temperature change of the space affects the temperature change of the pyroelectric element, the detection accuracy of the sensor may be unstable.
During screen printing, a squeegee presses and slides on the screen to apply a paste previously provided on the screen onto the pyroelectric element through a mesh. It is difficult to supply a specific amount of paste. Also, blurring may occur at a portion between the screen and the pyroelectric element. As a result, the printed pattern may have larger area than desired. If the area of upper surfaces of the supporting portions increases, a contact area with respect to the pyroelectric element increases. The increase in the contact area may cause heat conductivity to be uneven between the pyroelectric element and the supporting portions, and the detection accuracy of the sensor may be unstable.
The above-mentioned problems regarding heat increase as the size of the infrared sensor decreases. Since the conventional infrared sensor has been typically mounted on a relatively large product, such as an intrusion detector or a fire detector, the miniaturization of the sensor has not previously been highly desirable. However, in recent years, the sensor is desired to be mounted in a small product such as wireless equipment. Thus, the size of the infrared sensor must be further decreased. As the miniaturization continues, the dimension of the space between the base and the pyroelectric element, and slight variation of a bonded area of the supporting portions with respect to the pyroelectric element may seriously affect the detection accuracy.